The Muscle

Muscle Spindle and Reflex Function

• consists of two major parts

1. One motor neuron
2. A bundle of muscle fibers

• In order for us to have movement, the brain tells the motor neuron that there needs to be muscle contraction. The motor neuron stimulates the muscle fibers its associated with and the muscle contracts
• the motor neuron in the motor unit is called the alpha motor neuron
• In the following image there are 3 motor units (each a different colour, but all consisting of a nerve and its muscle fibers)

• each muscle fiber has intrafusal and extrafusal components
• the extrafusal part is contractable (can change its length when stimulated)
• stimulated to contract by alpha motor neurons
• the intrafusal part is not contractable at the center, but their ends are contractable
• the center is surrounded by nerves called 1a afferent neurons that sense stretch
• the ends of the intrafusal parts are stimulated to contract by gamma motor neurons

Important Components

1. Muscle Spindles - responsible for monitoring and responding to the length/stretch of the muscle
2. Golgi Tendon Organs - responsible for monitoring the tension in the muscle

1. alpha motor neuron - causes contraction of extrafusal fibers
2. gamma motor neuron - causes contraction of intrafusal fibers
3. 1a afferent neuron - senses stretch in the intrafusal fibers
4. 1b afferent neuron - senses tension in the tendon of the muscle

• located in the muscle body
• nerves associated with the spindle

1. alpha motor neuron
2. gamma motor neuron
3. 1a afferent neuron

• the sensory information about muscle stretch is coming from 1a afferent neuron. This is located in the intrafusal fibers.
• there is NO AFFERENT (SENSORY) neuron associated with the extrafusal fibers

• detects tension during contraction (in understandable terms, it detects the heaviness of something)
• located in the tendon
• protects us against lifting things that are too heavy for our muscles and causing injury.
• relays its information to the 1b afferent sensory nerve which passes on info to the CNS
• 1b sensory information has the ability to inhibit the alpha motor neuron, thus inhibiting muscle contraction and protecting us from harm.

Big Picture - The Stretch Reflex

1. the muscle stretches, activating 1a afferent nerve
2. the interneuron connects the 1a afferent nerve signal to the alpha motor neuron response
3. alpha motor neuron fires, causing extrafusal muscle contraction
4. this extrafusal contraction stimulated the gamma motor neuron to contract the intrafusal fibers
5. the contraction of the muscle body causes increased tension on the muscle tendon, activating the golgi tendon organs
6. golgi tendon organs activated 1b afferent neurons
7. the signal causes inhibition of alpha motor neurons ––> the muscle relaxes